1. Field of the Invention
The present invention relates to a key telephone apparatus.
2. Description of the Prior Art
Among a number of functions equipped in a telephone, there is an auto-redialing function. This function is used, for example, in a case of hurry so that when the line of the called station is busy, the telephone at the calling station automatically redials the number of the called station again and again until the line is connected. For some telephones, such as an auto-redialing function is equipped in the telephone itself, but for the key telephone devices incorporating microcomputers, the auto-redialing function is equipped in a main device having the switching function. An example of such a key telephone apparatus is explained below.
Referring to FIG. 5, a prior art key telephone apparatus is shown in which reference number 51 is a main device, and 52 is one of a plurality of key telephones connected to main device 51.
Provided in each key telephone 52 are a key telephone circuit 53, a handset 54 and a handset circuit 55 having a voice amplifier, etc. A reference number 56 is a 2-line/4-line inverter, 57 is an LED display for displaying various operating conditions, 58 is a control panel having a number of key buttons to select operation, 59 is a data transmission and receiving circuit for effecting the data transmission and receiving with the main device 51, and 60 is a hook switch turned on and off by handset 55.
In the main device 51, 62 is a key telephone system unit control circuit including a microcomputer, 63 is a data transmission and receiving circuit for effecting the data transmission with a key telephone 52, 64 are station circuits provided for each extension telephone, 65 is a trunk circuit for effecting the capturing or releasing the telephone network line, such as a trunk line, and for detecting the called signal, 66 is a switching network for establishing a channel between the trunk line and the extension, 67 is a dial signal sending circuit for sending a dialing signal to the trunk line in accordance with the signal from control circuit 62, 68 is a dial memory for storing telephone number data for auto-dialing or for temporarily storing telephone number data for auto-redialing, and 69 is a tone detection circuit for detecting the tone signal, such as a line busy signal. The detected tone signal will be applied to control circuit 62. Tone detection circuit 69 can be connected to trunk circuit 65 through switching network 66.
According to the prior art key telephone apparatus described above, the transmission between the main device 51 and each key telephone is carried out by two lines, voice line for sending voice signal and data line for sending control data. By this arrangement it is possible to exchange data frequently between the key telephone 52 and the main device 51. Usually, many function are equipped and centered in the main device 51 so that during operation, each key telephone sends a function request signal, such as an auto-redialing function request signal, through the data line to the main device in which the microcomputer in control circuit 62 takes care of the requests as it receives the request signals from number of key telephones in a timesharing manner.
A further explanation on the auto-redialing function will be given. When the main device 51 receives from one key telephone a data representing the telephone number of the called station and the auto-redialing function request signal through the data line, control circuit 62 operates dial signal sending circuit 67 to send out the dial signal to the trunk line. When the dial signal is sent out to the trunk line, switching network 66 is operated to connect the tone detection circuit 69 with trunk circuit 65. Thereafter, control circuit 62 detects the sound signal transmitted from tone detection circuit 69. If the detected sound signal is a line busy tone signal, control circuit 62 redials the same number after a predetermined time interval. If, on the other hand, the detected sound signal is a ringing signal, the control circuit 62 controls switching network 66 to break the connection between the tone detection circuit 69 and the trunk circuit 65 and, also to make a connection between one station circuit 64 and trunk circuit 65.
According to the above described prior art arrangement, since the tone detection circuit 69 is provided in the main device 51, the detection of the line busy signal is carried out in the microcomputer of the main device 51. This will result in overload work of the microcomputer, as explained below.
Since the microcomputer in the main device 51 receives various request signals from number of key telephones in a time-sharing manner, the detection of the line busy signal must be carried out within the corresponding time slot assigned to each key telephone. However, if the number of key telephones connected to the main device increases, or if the number of function available to each key telephone increases, work assigned to the microcomputer increases. Under such a condition, the detection of the line busy signal may be an over burden to the microcomputer.
Alternatively, if it is so arranged that switching network 66 connects the tone detection circuit 69 with the trunk circuit 65 without any control by control circuit 62, the line busy signal may be detected by the tone detection circuit 69, but such a line busy signal will not be sent to the key telephone. In this case, the trunk line will be connected to the key telephone only after the detection of the ringing signal. Therefore, in this case, the user of the key telephone will not here the dialing sound or line busy signal. Thus, the user who is used to use the conventional type telephone will feel some anxiety, particularly when the line is busy, whether or not the telephone is operating properly, until he/she hears the ringing signal.
In consideration of the above, the speaker circuit of the extension key telephone may be connected with the tone detection circuit 69 and have its own line busy signal detection circuit. This arrangement will result in reducing the load of the microcomputer in the main device 51, and also will enable the user to hear the line busy signal or the ringing signal.
However, there is still a problem, even after the employment of the line busy signal detection circuit in each extension key telephone, such that an exact time when the dialing signal has completely sent out to the trunk line can not be detected, because the sending of the dialing signal is done within the main device 51. Therefore, the detection of the line busy signal, which should be carried out after the dialing signal has been sent, will not be carried out at a desired timed relation with the dialing.